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Will is attempting to send out an SOS using balloon-assisted rockets. Moments after the Robot launches the latest rocket, something goes terribly wrong and the rocket ignites the hydrogen filled balloon and plummets to the ground in a fireball. At the last moment Will and his sister Penny are pulled out of the way by their father. Later on, Don finds some fuel is missing and blames Smith, but it turns out that Will has used far more than he thought in his experiments.

Short of fuel, John and Don come up with an idea to build a plasma engine to power the Jupiter 2. Their initial experiments are encouraging. The Robinsons are eager to resume their flight to Alpha Centauri, although Dr. Smith wishes to return to Earth. The first real test of the modifications to the Jupiter 2's engines are a failure, though, and all of the crew are quite depressed at the outcome.

Will comes up with a new idea—rather than send the Jupiter 2, why not build a "raft" big enough for two to send to earth. With a balloon assisted take-off, it seems feasible and the crew begin adapting the ship's reactor core for use as a small spacecraft.

The family works night and day on the Reactor Unit. After days of intensive work the ship is finally ready for lift-off.Although Don is set to pilot the small craft to Earth, Dr. Smith accidentally gives the lift off signal to the Robot while he and Will are aboard. The Robinsons are very concerned over their son's unplanned trip, but have confidence that he will know what to do. All goes well and a planet appears on their radar, but they are unable to raise Alpha Control or any other station on Earth.

They make land in a desolate landscape; Dr. Smith is certain they are in the Badlands of the Dakotas or perhaps Wyoming, but Will is not convinced. They soon meet a two-headed plant creature; even then Smith insists it is merely a skunk-cabbage. Will grows increasingly worried now that he is positive they are not on Earth; he reaches out for Dr. Smith, saying he hopes that he will be a father to him just like his own father; as they set off, Smith makes the 'fatherly' gesture of offering to give Will a piggy-back ride. They soon come to a dead end and find the alien creature has blocked their only way out.

At the Jupiter 2 everyone is concerned at the lack of further radio contact from Will when they pick up a blip on the radar matching the reactor core's signature. They realize that the raft must have landed elsewhere on their planet within range of their radar. John embarks in the chariot and Don with the jetpack to find Dr. Smith and Will. Meanwhile, Will tries to communicate with the plant creature; when Dr. Smith begins eating berries from another plant, it grabs him and Will desperately tries to reach his father by radio. John picks up the signal and is able to home in on Will and Smith's location. He arrives just in time to shoot the two-headed creature as it is advancing menacingly on his son and Dr. Smith.

John later explains that the mission failed because the raft simply could not generate enough power to escape the planet's gravitational pull, even with the new plasma drive.

The first time where Dr Smith calls the Robot a "ninny", when Smith is trying to find out if the Robot can launch the raft.

The reactor core the castaways used to create the raft, looks suspiciously like the Diving Bell from Voyage to the Bottom of the Sea (also an Irwin Allen production). It will also appear in various forms throughout the series in such episodes "The Sky Pirate," "A Change of Space," and "Rocket to Earth."

Dr. Smith's "skunk cabbage" monster looks a lot like an undersea monster that menaced the Seaview in an episode of Voyage to the Bottom of the Sea.

This craft does not have the same lines as the space pod introduced in the third season, however, the space pod was probably trapped inside the Jupiter 2 each time they crash landed in seasons 1 and 2. It does introduce a nit - if they can get the reactor core out through the limited openings available, why can't they unload the space pod?

Professor Robinson devises a way of making 'unlimited propellant' from a plasma reaction in this episode. Despite initial tests working well, the use on the actual engines backfired - and the Space Raft did not have enough power to escape the orbit of Priplanus. It is strange that the Space Pod later has no trouble doing the same later on, but of course this spacecraft was not thought up yet. Also, the new invention of the plasma fuel is never again referred to, possibly so that the plot point of the Robinsons running out of fuel continually could keep being used.

With respect to scientific terminology, lost in space seems to be confusing the terms "propellant" and "fuel." Technically propellant is the material used to generate thrust, not fuel.

The decontamination procedure is actually accurate if the contamination is in the form of radioactive dust particles inside the raft. However, the run-off would have to be collected and deposed of.

In the opening scene, the small balloon lifts the tiny rocket ship. When it crashes back to Priplanus, the force field saves the Professor, Will, and Penny. We are then told that the force field only works in one direction. It allows objects to leave, but does not allow objects to approach. This contradicts the behavior of the force field in "There Were Giants in the Earth", when the Robot turned off the force field in order to leave the campsite.

When Smith and Will are in the raft travelling through space, they rock gently back and forth to mimic the motion of their little spaceship. At one point Bill Mumy's rocking motion gets out of sync with Jonathan Harris', so Mumy stops the rocking motion until he can resynch with Harris' rocking motion, and then continues.

Again, like Hapgood's spaceship in "Welcome Stranger," there seem to be no provision for air, water, or food on the Raft.

Why does Don get so darn angry when Doctor Smith calls them “galactic castaways”? It’s not an insult... it’s accurate!

If the ship’s reactor core has been used to make a space raft, what is all the radioactive material being stored in now? If the core used to have radioactive material in it, wouldn’t it be very dangerous to get inside? They seemed to spray it out a bit, but it's doubtful that would have cleaned it completely.

When stuck in the vine-cage, Will sees some alien fruit and immediately starts eating it. Didn't Will learn from the episode "The Oasis" what terrible things can happen when one eats strange fruit before it's been tested?

This episode obviously uses a great deal of stock footage in order to save production costs.

Note that the Robot begins his journey into eccentricity here by insulting Doctor Smith for the first time.

In the opening scene, just prior to the Robot releasing the rocket, you can see the wire on top of the balloon that will lift it skywards.

As Will and Dr Smith peer out of the Raft’s porthole after the Robot release the raft’s rope tie-downs, the camera view is all wrong. We see the top of the Robot’s bubble as the ship ascends, but Will and Smith appear to be looking out, not down.

John checks the raft for contamination after a fully suited up Don has earlier entered the vessel and sprayed the crap out of it. John leans right in and tests it, but unfortunately, he’s not wearing any protective gear. He even says to Don that there’s still a bit of contamination remaining!!